Nowadays, an image scanner has gradually become standard peripheral equipment of a personal computer. Therefore, scanner manufacturers have been trying their best to improve the scanning quality, for example, to compensate for brightness, chrominance, etc., and/or correct scanning line errors. On the other hand, high scanning speed is also a criterion of an exquisite image scanner.
In order to enhance the scanning speed of an image scanner, several devices and methods have been proposed in prior art. For example, please refer to FIG. 1 which schematically shows home-sensor means for speeding up the movement of an image pickup module 11 from a standby line N to a scan start line M, i.e. a document reference line. In this case, it is assumed that the image pickup module 11 reaches the scan start line M after moving a distance d from the standby line N along the arrow direction. In other words, the image pickup module 11 directly moves a predetermined distance, rather than moving pixel by pixel to detect the presence of a document, and then starts scanning. Therefore, the movement of the image pickup module 11 from the standby line N to the scan start line M is fast.
However, there are likely to be defects resulting from errors of various parts and/or assembling inaccuracy in this home sensor means. For example, if the image pickup module 11 is inaccurately installed at a position lower than a predetermined one while assembling, i.e. the standby line is lowered, the actual scan start line M1 will be accordingly lower than the document reference line M after the same distance d of movement, referring to the dotted lines and the dotted arrow in FIG. 1. Therefore, the top portion 13 of the document 12 will be missed in the scanning operation.
On the other hand, when the image pickup module 11 reaches the scan start line M, the nth pixel unit of a CCD of the image pickup module 11 generally serves as the start point of the same scanning line, and the nth pixel unit and the pixel units thereafter are used to simultaneously pick up the image of the document line by line. If the image pickup module 11 deflects from its predetermined path owing to an assembling defect, the same distance d of movement will make the image pickup module 11 reach another line rather than the predetermined scan start line M so that the scanning lines will slant. Therefore, the scanning quality will be adversely affected.
Another conventional means is proposed to avoid the missing of document data. Please refer to FIG. 2 which schematically shows means for precisely determining a scan start point in an image scanner. On the scanning platform of this image scanner, a black region 21 is provided in front of a scan start line R where a front edge of a document 22 to be scanned is positioned. In this case, the image pickup module (not shown) quickly moves toward the black region 21, and then slowly passes through the black region 21 after reaching the black region 21 to detect the disappearance of the black color. When the detected black color disappears, the image pickup module keeps on moving and begins to measure a distance Q along the arrow direction. It is assumed that the scan start line R is reached after the image pickup module moves the distance Q from the reference point P. In this way, the downshift error resulting from the downshift installation of the image pickup module, as shown in FIG. 1, can be avoided because the predetermined distance Q is measured from the lower edge of the black region, i.e. the reference point P, rather than from the standby line of the image pickup module. Nevertheless, the defect of the conventional scanner resulting from the deflection of the scanning lines still exists in this means, and is not resolved.